1). Field of the Invention
This invention relates to a wafer transfer cassette.
2). Discussion of Related Art
Integrated circuits are manufactured in or on silicon or other semiconductor wafers. During processing of a wafer one or more steps involves inserting the wafer into a processing chamber which is maintained at a relatively high temperature and depositing a layer on the wafer.
A plurality of wafers are usually located one above the other in a transport cassette which is then positioned adjacent a factory integration unit. A robot within the factory integration unit transfers the wafers one by one through the factory integration unit to a load lock. A wafer transfer cassette is located within the load lock and the wafers are located one above the other in the wafer transfer cassette. A door is then closed between the factory integration unit and the load lock, whereafter a slit valve between the load lock and a transfer chamber is opened. The wafers are then transferred from the wafer transfer cassette through the transfer chamber into a processing chamber. The wafer is heated within the processing chamber which may be maintained at a temperature of more than 800.degree. C. The wafer is then transferred, sometimes via a cool-down chamber, to the transfer chamber, and from the transfer chamber back onto the wafer transfer cassette located within the load lock. When the wafer is located on the wafer transfer cassette, its temperature may be as high as 200.degree. C.
Because of the heat of the wafer, the wafer transfer cassette is usually subjected to the high temperatures from the wafer. The wafer transfer cassette is also subjected to corrosive chemicals passing from the processing chamber through the transfer chamber to the load lock. The wafer transfer cassette therefore has to be designed with these conditions in mind.
One typical wafer transfer cassette comprises a frame having first and second vertically extending support structures that are horizontally spaced from one another. Fins are provided on surfaces of the support structures. Each support structure and its fins are integrally manufactured in a casting or machining operation from a single material such as aluminum or stainless steel. A metal such as aluminum or stainless steel has superior strength characteristics when compared to, for example, a plastics material, thus providing the required strength to the wafer transfer cassette. Metal fins, however, give off particles on lower surfaces of wafers that they support, especially in corrosive conditions. These metal particles can diffuse through the wafer and destroy an integrated circuit formed thereon.
Other wafer cassettes, typically wafer transport cassettes that are used for transporting wafers from location to location in a fabrication environment, have support structures and fins that are integrally formed in a casting operation from a plastics material. Thus far the industry has shied away from using plastic wafer transfer cassettes because of inferior structural and thermal characteristics because, it is believed, that fins on such wafer transfer cassettes would buckle or bend when a wafer is located thereon and the fins are subjected to high temperatures. Buckling and bending is particularly problematic when larger, heavier wafers are located thereon.